This invention relates to the preparation of finely divided normally solid, synthetic organic polymeric thermoplastic resins.
Thermoplastic polymers in powder or finely divided form have a wide variety of commercial applications, such as for example, the dry powders have been used to coat articles in dry form by dip coating in either static or fluidized beds, by electrostatic coating, spraying, or dusting and flame spraying. The powders are used in dispersed form in suitable liquid carriers to apply coatings by roller coating, spray coating, and dip coating to a variety of substrates such as, glass, ceramics, metal, wood, cloth, paper, paperboard, and the like. The finely divided polymers have also been successfully employed in conventional powder molding techniques. The fine powders have also been applied as paper pulp additives, mold release agents, wax polish, paint compositions, binders for non woven fabrics and finishes for woven fabrics.
There are basically four types of processes employed in the prior art for preparing finely divided polymer particles, i.e., mechanical grinding, solvent precipitated, dispersion and spray atomization of solutions or slurries.
Generally mechanical grinding employs conventional equipment to yield particles of irregular shape and wide size variation of from about 75 to 300 microns. The powders produced by this method may not be suitable for applications where free flowing powders are required, since the irregular shapes may inhibit the flowability of these powders. The grinding of some polymer may be very costly because of the toughness of the resin even when cryogenically cooled.
The spray techniques are generally satisfactory for producing uniform non-agglomerated spherical particles, however very specialized equipment, usually nozzles operating under a limited range of conditions to prevent nozzle plugging are required. Substantial problems in spraying are the shearing of a polymer solution as it passes through the nozzle, premature precipitation of the polymer or too rapid volatilzation of solvent.
The dispersion method also is subject to high shear conditions. Frequently water is the dispersing medium and dispersing agents are used to facilitate the dispersion. Hence the powders produced by this technique generally incorporate some or all of the dispersing agent in the powder which can create undesirable changes in the original polymer properties, e.g., increased water sensitivity, loss of electrical insulating values, loss of adhesive capabilities, etc.
The final type of prior art process generally entails dissolving the polymer in a solvent, followed by precipitation of the polymer in finely divided form through addition of a nonsolvent, evaporation of the solvent or a combination of the two. Problems in this process have included difficulty in manipulating the solvents, solvent removal, particle agglomeration which requires a great deal of grinding, and particles having irregular somewhat rounded shapes.
The solvent system method, however, is a relatively simple procedure for producing powders for many applications and there are a number of patents, relating to improvements. For example, U.S. Pat. No. 3,563,975, which discloses the preparation of high density polymer powders by cooling a hot solution of polymer under sufficient pressure to prevent substantial vaporization of the solvent. The patentee recognized that shear stress occurring just before the polymer is precipitated, cause polymer strings. To prevent this undesirable occurence an elongated cooling means was devised to precipitate polymer particles. The elongated cooling means operated with intermittent high velocity movement of material therethrough, which resulted in very little turbulence.
In another process described in British Pat. No. 1,172,317 particulate powder was precipitated in a quiescent solution to avoid shear stresses and the resultant polymer strings.
Both of these processes are primarily dependent on non-turbulent precipitation of solutions during cooling, which substantially enhanced the problem of heat removal. An agitated precipitation would be far easier to cool and hence would require simplier equipment and techniques and less time and expense than those of the prior art.